Sinueretone A, a Diterpenoid with Unprecedented Tricyclo[12.1.0.05,9] pentadecane Carbon Scaffold from the South China Sea Soft Coral Sinularia erecta

A novel diterpenoid, sinueretone A (1), featuring an unprecedented tricyclo[12.1.0.0(5,9)]( )pentadecane carbon framework, along with two new (2 and 3) and one known (4) casbane diterpenoids were isolated from the South China Sea soft coral Sinularia erecta. The structures of the new compounds, especially their absolute stereochemistry, were established by extensive spectroscopic analysis, various quantum chemical calculations, and/or X-ray diffraction analyses. A plausible biogenetic relationship of 1 4 was proposed, which gave an insight for future biomimetic synthesis of the novel compounds. In a bioassay, compounds 1 and 2 displayed interesting anti-inflammatory activity by the inhibition of lipopolysaccharide-induced tumor necrosis factor-alpha protein release.